I wonder if the more analytical of our members would be able to interpret the following statistics, harvested from a ZyXEL VMG1312-B10A that is acting as the CPE on a long metallic pathway, and is configured to operate only in ADSL2 (ITU-T G.992.3) mode. In particular, do the error counts hint at any possible problem? > xdslctl info --showxdslctl: ADSL driver and PHY statusStatus: ShowtimeLast Retrain Reason: 8000Last initialization procedure status: 0Max: Upstream rate = 417 Kbps, Downstream rate = 3428 KbpsBearer: 0, Upstream rate = 445 Kbps, Downstream rate = 3082 Kbps

You look as if you are having error bursts with 4% of Errored Seconds being SES... so I'm betting the problem is short lived and repetitive - have you got a ES graph?

I'm having experiments with two lines one of which has an SNR of 3 and the other of 11. There's a surprisingly similar pattern on both lines despite the SNR difference. Your interference may be similar if the uptime is low (and have the same low chance of sussing it out if it is similar ).

No, those harvested statistics do not relate to my circuit . . . but were "pushed under the door of The Cattery" early yesterday morning, with a note asking about the error counts. The only comment I could make was that without details of the relevant time-frame, etc, not a lot could be deduced.

When I did that I had got the idea from a suggestion by Burakkucat. He thought that forcing a restriction to ADSL2 on a very long line may close the modem to shut its ears to the very high tones used by ADSL2+, thus narrowing the window and letting less noise in. I don’t know whether or not modems do this; it could be that they have the same input subsystem and there is a variable width input filter or the hardware could be identical. I assumed that it couldn’t hurt and so I tried the idea when I first went up to ADSL2 from G.992.1. Indeed the highest tones I can use are way too low for me to lose out on anything by using ADSL2 not ADSL2 A+.

One difference which I noticed last night, is that if you have PhyR / G.998.4 / G.INP, for ADSL2 there are differences in the standards compared with ADSL2+ (and VDSL2). See G.998.4 annex A.

For ADSL2, the maximum amount of RAM to be allocated for the ReTX Tx queue in the DSLAM, quoting - "transmit retransmission queue in the CO" - is limited to the half of the downstream interleaver delay in bytes, i.e.: Qtx* Q * H ≤ 8001 bytes". For ADSL2+ this figure can optionally be 12000 bytes instead - see annex B. The maximum value of Q*H is 1024 bytes, but I can’t see all the individual parameters for my modem.

So a constraint on performance is possibly relaxed with ADSL2+, by an implementation option. This will affect the range of patterns of lookback retransmission that can be supported, where a receiver can indicate a NACKed or ACKed DTU way back which is different from its successors. When such a circumstance could practically happen is quite unclear to me, and even the mere possibility is an option.

A more careful look might reveal other differences between ADSL2 and ADSL2+, but I have yet to do all the tedious work.